It's easy, and indeed trivializing, to blame society for the claimed 'pitting women against eachother'. To be sure, there are plenty of cultural factors that reinforce competition between women, but men and women need no encouragement to compete to get a mate. As soon as one person is attractive to more than one other person (regardless of the gender of anyone) those who are attracted are in competition. This isn't just logic, it's zoology, and dumping all this gender politics on it just gives people an excuse to think of themselves as victims.
This kind of competition is an inevitable part of mammalian existence. To be human is to struggle against our mammalian baggage when it is harmful to ourselves and to others.
If there really was a war between all the men and all the women, then divide and rule would be an effective tactic and a meaningful accusation. But most people, regardless of their gender and gender preference, want to get with someone, for reasons of friendship, companionship, comfort and joy, of which physical relations are an (almost) inevitable part.
This said; the first image reinforces competitive behaviours by drawing comparisons. The second implies a comparison, however - it implies that if your thighs don't touch, you can't be as attractive as Marilyn.
All this said; if the two women in the first image were (unknown to me, and) competing for my interest, I literally wouldn't know where to put my head. Both are very fine examples of young women in excellent health (a sign of reproductive fitness), who take care over their appearance (a sign of good self-esteem), and whose posture is relaxed and confident (a sign they will be easy to get along with). They may be very different examples, but I if they were competing for me, I wouldn't base my response on what I observed from a photograph anyway.
A final point: Marilyn was adored by a lot more than thousands of men, but not for hear appearance. As Ms Cromarty observes, she was a very good actor; funny and quirky, and she didn't need to pout or flash her cleavage to compete on set with the boys. But most of all, she had air of vulnerability that made her seem accessible. Everyone is vulnerable; we form friendships, couples, families, to help us to feel more secure, to help us to support one another.
It has too often been an aim of feminism to toughen women up, to try to make you impervious, invulnerable, so that you won't be a victim of all the harmful influences of a male dominated culture. This attitude however, will not help you with all the problems that come from within, come from the fact of being a human mammal. Furthermore, it will make it harder for you to make friends, find a mate and raise children. The strongest alliances are between people who know their own, and eachother's, weaknesses. To form them we have to show our vulnerabilities.
Awareness of your own weaknesses, your own vulnerabilities, is therefore the best defence against the harmful influence of our culture. The woman who sees Marilyn and does not know how susceptible she is to her own desire to compete, will compare herself physically to Marilyn and find herself wanting. The woman who is aware of this susceptibility, however, will remind herself that Marilyn is Marilyn, and has Marilyn's unique reasons for being attractive, that go way beyond the way she looks; and she will remind herself that she has her own reasons for being attractive that also go way beyond the way she looks, that she doesn't need to be, any more than she can be, attractive to everyone. Just attractive to someone.